Recently, development in the digital technology has lead to popularization of capturing high-resolution digital moving images using digital cameras and digital video cameras. In general, the digital moving image is compressed (encoded) to be efficiently recorded in a recording medium such as a flash memory. A widely-used coding method for the moving image is H.264/MPEG-4 Advanced Video Coding (AVC), i.e., H.264 (refer to NPL 1).
Recently, activities have been started for internationally standardizing a successor coding method of H.264 having higher efficiency. More specifically, Joint Collaboration Team on Video Coding (JCT-VC) has been established between ISO/IEC and ITU-T. JCT-VC is developing High Efficiency Video Coding (HEVC) as a standard.
In standardizing HEVC, various coding methods are being discussed in terms of simplifying implementation and shortening processing time, in addition to improving the coding efficiency. One of such methods is referred to as a Tile technique which divides one frame into a plurality of rectangular regions, so that coding and decoding can be performed in parallel (refer to NPL 2).
By employing the Tile technique and performing parallel processing of coding and decoding, high speed processing can be realized, and memory capacities of the image coding apparatus and the image decoding apparatus can be reduced. Further, the Tile technique is also employed in a JPEG 2000 coding method (refer to ITU-T T.800 (08/2002) JPEG 2000 Image Coding System Core coding system).
Furthermore, a preview (or a thumbnail) function of still image and moving image contents is widely used so that a user can easily recognize the content of the digital image. The preview usually uses a reduced image of the entire frame.
However, in recent years, the numbers of pixels in still images and moving images have increased. It is thus desirable that a mode which realizes the detailed display of only a portion of the content as a Region-of-Interest (ROI) is usable. More specifically, since the content cannot be sufficiently expressed in detail on a screen of a limited display size, e.g., in a mobile terminal, such a mode is desirable.
Further, important information (e.g., a main object such as a person) in the content captured at a wide angle may exist in a region corresponding to only a portion of the frame. In such a case, it is desirable to enlarge the important information and perform high-definition display as the preview.
Furthermore, it is desirable that the preview is processed at maximum speed in terms of user-friendliness. It is thus desirable to decode only the tiles corresponding to the portion of the frame to perform high-speed processing, and display as the preview.
In other words, the decoding time is greatly shortened by decoding only the tiles, as compared to decoding the entire frame of the still image or the moving image and displaying the preview. According to the present exemplary embodiment, the tiles displayed in previewing will be hereinafter referred to as preview tile.
PTL 1 discusses a technique which decodes, when displaying a portion of the frame, up to a high frequency component of the tiles to be displayed, and displaying a high image quality preview.
However, the technique discussed in PTL 1, i.e., the Tile technique in HEVC, does not include an image coding format (hereinafter referred to as a coding format) in which the preview tile is determined among the tiles in the frame. More specifically, when the tiles corresponding to a portion of the frame are displayed as the preview, the tiles in a compressed coded stream to be used for displaying the preview are not defined.
If the tiles corresponding to only a portion of the frame are to be displayed as the preview as described above, it is necessary for the decoding apparatus or a decoding program to independently determine the tiles to be used. As a result, the content of the preview may become different between the decoding apparatus and the decoding program.
Further, when a content creator displays a portion of the frame as the preview, it is desirable for the content creator to designate the content of the preview. If the preview is appropriately designated, a viewer of the content becomes capable of selecting and searching the content without hesitation.
However, the content creator cannot designate the preview tile using the coding format in the technique discussed in PTL 1. Further, if the information on the tiles to be used as the preview is included as user data in the coded stream, the decoding apparatus and the decoding program independently determine whether to use the user data, so that the desired preview may not be acquired.
Furthermore, the technique discussed in PTL 1 is the decoding method of the tiles used for displaying a display region at higher image quality when the region to be displayed is predetermined. The method is thus not a method for designating the tiles to be displayed when performing previewing.